PROJECT SUMMARY Chronic exposure to nicotine in tobacco products results in numerous health consequences (lung cancer, emphysema, hypertension, etc.) and accounts for over 6 million deaths per year. Relapse rates are high among those who attempt to quit smoking, and pharmacotherapies that seek to foster smoking cessation have limited effectiveness. Thus, there is a significant unmet need for more effective strategies to treat nicotine dependence. Nicotine exposure produces physical dependence, and the physical and/or emotional nicotine withdrawal symptoms ? as compared to the rewarding effects of nicotine ? are often the most important contributors to relapse. Unfortunately, few research studies have probed the important question of physical dependence and nicotine withdrawal mechanisms. Indeed, a critical gap in knowledge exists regarding our understanding of how chronic nicotine exposure establishes physical dependence and therefore makes smokers highly susceptible to relapse. In this project, we will use mouse models to study the medial habenula (MHB), a small brain area in the epithalamic region that has recently been implicated in nicotine withdrawal, and which expresses extraordinarily high levels of several types of nicotinic acetylcholine receptors (nAChRs). nAChRs mediate the psychoactive and addictive action of nicotine, and we intend to identify the relevant nAChRs and MHB circuits involved in nicotine dependence and withdrawal. Three independent and complementary AIMs are proposed, each of which probes a specific mechanistic aspect of the response to nicotine in the MHB. In AIM 1, we will use biophysical and optical techniques to determine through what mechanisms acute nicotine differentially activates specific cell types in MHB. In AIM 2, we will employ similar techniques to determine how chronic nicotine selectively enhances neuronal activity in a specific sub-circuit of the withdrawal pathway. Finally, in AIM 3, we will couple physiology techniques with a novel behavioral/systems approach to identify important MHB circuits involved in generating physical and/or emotional responses during nicotine withdrawal. Together, these AIMs will help us solve the problem of understanding how cessation of nicotine intake causes the brain to generate aversive physical and emotional withdrawal responses that inevitably lead to relapse. Solving this problem could lead to new strategies or drugs to foster smoking cessation.